During normal operations of a liquefied natural gas (LNG) carrier, LNG cargo being transported in LNG storage tanks boils off as gas, hereinafter referred to as “boil off gas (BOG)”. This boil off is due, in part, to ambient heat ingress through the tank insulation. In addition, energy is imparted to the LNG by the dynamic motions of the vessel itself, again resulting in boil off.
Cargo tank pressures, and thus cargo temperatures, are managed during a sea going voyage by removing this BOG. Typically, this is done using the vessel's gas compressors and other machinery wherein the BOG is used as a fuel for propulsion and power generation. Due to external conditions, such as rough seas, the amount of BOG that has to be removed from the LNG storage tanks sometimes exceeds the requirements and capacity of the ship's machinery to use the BOG. In this case, the surplus energy in the BOG is typically removed overboard such as by a steam dump system, in the case of a steam propelled vessel, or by a Gas Combustion Unit (GCU) if the vessel is propelled by a dual fuel diesel electric (DFDE) drive.
Tassel, United States Patent Application No. 2006/00538806 describes the use of a system including a buffer tank to temporarily store BOG at low pressures, i.e. 20 to 30 psig, prior to combustion by prime movers. However, this particular system has a number of shortcomings.
First, only a single stage BOG buffer tank is utilized to store BOG. Second, the pressure in BOG downstream from the buffer tank cannot be increased if necessary to meet delivery pressure requirements of equipment used to combust the BOG. The pressure in the system is maximum in the single stage BOG buffer tank and no accommodation is made for further increasing the pressure of the BOG other than pumping additional BOG into the BOG buffer tank to increase pressure.
With current concerns relating to environmental impacts of any type of emissions on the environment, it is desirable to have a more efficient method of using and storing BOG than is conventionally employed by LNG transport vessels.